Conventional borehole survey technology involving the use of either magnetic or gyroscopic instruments has provided at best a lateral position uncertainty of 1% of the measured well depth. The conventional instruments also exhibit severe accuracy degradation in inclined boreholes, even though some recent mechanical instruments use modern inertial navigation and guidance techniques. As outlined in, "Analysis of Alternate Borehole Survey Systems" by S. Brezkowski and J. Fagan in the Journal of The Institute of Navigation, Vol. 30, No. 4, 1983-1984, page 309-324, such mechanical instruments are difficult and expensive to adapt to the severe environment at the downhole end of a drill string. Details of some such instruments are provided in U.S. Pat. Nos. 4,265,028 entitled "Survey Apparatus And Method Employing Canted Tilt Sensor" by Van Steenwyk, 3,753,296 entitled "Well Mapping Apparatus And Method" by Van Steenwyk, 4,245,498 entitled "Well Surveying Instrument Sensor" by Poquette, Jr., 4,192,077 entitled "Survey Apparatus And Method Employing Rate-of-Turn And Free Gyroscopes" by Van Steenwyk, et al, 4,199,869 entitled "Mapping Apparatus Employing Two Input Axis Gyroscopic Means" by Van Steenwyk and 4,197,654 entitled "Survey Apparatus And Method Employing All Latitude, All Attitude Gyrocompassing" by Van Steenwyk, et al. Since such devices are mechanical, they require large amounts of power, and in adverse environments, tend to fail at inopportune times requiring the removal of the entire drill string, an especially expensive and time consuming process when deep wells are involved.
Therefore there has been a need for a bore hole survey instrument that can be placed adjacent to the drill head, can withstand the extreme environment to which such drill heads are exposed, is reliable, takes little power and can be constructed relatively economically and does not require that the surrounding drill string is non-magnetic.
Conventional mechanical gyro cannot survive the oil drilling environment so that accurate surveys can only be made by removing the entire drill string and then running gyroscopes, of the conventional mechanical spinning type, down the hole as a survey tool. The only way measurement while drilling is currently done is with magnetometers and they suffer from two major defects. Magnetic anomalies in the earth can cause errors that may be large in unfavorable locations, and in order for the magnetic steering tool to function, it must be encased in a non-magnetic drill section, usually stainless steel. Unfortunately stainless steel is brittle and has a tendency to shear with the resultant loss of the drill bit and instrument package. In addition, the cost of pulling up the string and sending it back down may be more than $1,000,000.